against the danger of overstressing by the posting of a single weight limit, if the limit posted corresponds to the load that may be safely carried on the shortest vehicle, such posting will prohibit the use of the bridge by longer vehicles of substantially heavier weight that could safely use it.
Since both the weight and length of vehicles in combination determine their effects upon bridges and since the weight-length relation varies so widely among all the vehicles that will use a bridge—and since, further, vehicles of different weights will have exactly the same effect if their lengths are varied in proper relation to their weights, there is no single answer to the question as to what weight of vehicle a particular bridge will safely support.
Conventional design loadings
In the design of highway bridges certain conventional vehicular loadings are assumed. The magnitude of the assumed loading is described by a designation involving the letters H or H and S and numerals expressive of weight. The combination expresses loading equivalent to that of a vehicle or combination of vehicles of definite weight and length. Thus, H15 loading is equivalent to the weight of a single vehicle of 15 tons total weight carried on two axles, 14 feet apart, one loaded with 12 and the other with 3 tons. H20-S16 loading is equivalent to the loading of a tractor-semitrailer combination of 36 tons total weight carried on three axles, separated by distances of 14 feet and loaded with 4, 16, and 16 tons, respectively. An H20-S16 bridge is a bridge designed for H20-S16 loading. An H20 bridge is a bridge designed for H20 loading.
But the fact that an H15 bridge is designed to support a vehicle of 15 tons or 30,000 pounds total weight does not mean that it will, with equal safety, carry every vehicle of 30,000 pounds total weight nor that a vehicle of 30,000 pounds total weight is the heaviest vehicle it will safely support. Such a bridge would be overstressed by a 30,000-pound vehicle having a wheel base of less than 14 feet; it would carry without any overstress a combination vehicle weighing 60,000 pounds having an over-all wheel base of 42 feet and axles appropriately loaded.
Fallacy of gross-weight control
Herein lies the fallacy of laws aiming to control the gross weight of vehicles by specified fixed limits of weight corresponding to certain classes of vehicles, such as 30,000 pounds for two-axle vehicles, 40,000 pounds for vehicles of three or more axles. Such limits are unnecessary for the protection of roads which, as previously explained, are affected by axle loading rather than gross load. They permit the operation of vehicles that will generate in some bridges stresses greater than those contemplated in their design, and more frequently, if they are enforced, they will prohibit the operation of many heavier but safer vehicles.
For example, a State in which there are many bridges of H15 design standard, if its laws limit gross weight at 30,000 and 40,000 pounds for vehicles of two and three or more axles, respectively, will permit the overstressing of its H15 bridges by vehicles of 30,000 pounds gross weight and wheel base less than 14 feet; and it will, supposedly at least, prohibit the operation of many vehicle combinations of the greater
